1. Field of the Invention
The present invention relates to a superheterodyne receiver, such as a television tuner or the like.
2. Description of the Related Art
A related art receiver, such as a television tuner, will be described with reference to FIG. 3. An input tuning circuit 2 is coupled to an input terminal 1 to which a television signal is input. The input tuning circuit 2 includes inductive elements 2a and 2b that are connected in parallel with each other, a capacitor 2c through which one end of the inductive element 2a is grounded, a varactor diode 2d, an anode of which is connected to the inductive element 2b, and a capacitor 2e through which a cathode of the varactor diode 2d is grounded. A connection point between the inductive element 2a and the capacitor 2c is connected to a ground through a resistor 3. In addition, the cathode of the varactor diode 2d is connected to a tuning voltage terminal 4. The input tuning circuit 2 is coupled to an RF amplifier 7 through a series circuit of a coupling varactor diode 5 and a coupling capacitor 6. An anode of the coupling varactor diode 5 is connected to the anode of the varactor diode 2d, and a cathode thereof is connected to the tuning voltage terminal 4.
A primary circuit of an interstage tuning circuit 8 coupled to an output of the RF amplifier 7 includes a varactor diode 8a, an inductive element 8b that is connected to an anode of the varactor diode 8a, and a capacitor 8c through which a cathode of the varactor diode 8a is grounded. A secondary circuit of the interstage tuning circuit 8 also includes a varactor diode 8d, an inductive element 8e that is connected to an anode of the varactor diode 8d, and a capacitor 8f through which a cathode of the varactor diode 8d is grounded. The two inductive elements 8b and 8e are connected in series to each other, and a connection point between the two inductive elements 8b and 8e is connected to one end of a coupling inductive element 8g. The other end of the inductive element 8g is grounded through a capacitor 8h. The cathodes of the two varactor diodes 8a and 8d are connected to the tuning voltage terminal 4. In addition, a connection point between the coupling inductive element 8g and the capacitor 8h is connected to the ground through a resistor 9.
The interstage tuning circuit 8 is coupled to a mixer 12 through a series circuit of a coupling varactor diode 10 and a capacitor 11. An anode of the coupling varactor diode 10 is connected to the anode of the varactor diode 8d, and a cathode thereof is connected to the tuning voltage terminal 4. A local oscillating signal is supplied from an oscillator 13 to the mixer 12. And then, a television signal output from the interstage tuning circuit 8 is converted into an intermediate frequency signal by the mixer 12. A resonating circuit 14 constituting the oscillator 13 includes a varactor diode 14a, a capacitor 14b through which a cathode of the varactor diode 14a is grounded, an inductive element 14c, one end of which is connected to an anode of the varactor diode 14a, and a capacitor 14d through which the other end of the inductive element 14c is grounded. The cathode of the varactor diode 14a is connected to the tuning voltage terminal 4. In addition, a connection point between the inductive element 14c and the capacitor 14d is connected to the ground through the resistor 15.
The voltage on the anode of the varactor diode 2d in the input tuning circuit 2, the voltage on the anode of each of the two varactor diodes 8a and 8d in the interstage tuning circuit 8, and the voltage on the anode of the varactor diode 14a in the resonating circuit 14 are switched by a transistor 16. An emitter of the transistor 16 is grounded, and a predetermined voltage is applied to a collector of the transistor 16 from a voltage stabilization circuit 17 through a protection resistor 18. To a base of the transistor 16, a switching voltage for turning on or off the transistor 16 is applied. And then, the connection point between the inductive element 2a and the capacitor 2c in the input tuning circuit 2 is connected to the collector of the transistor 16 through a resistor 19. Further, the connection point between the coupling inductive element 8g and the capacitor 8h in the interstage tuning circuit 8 is connected to the collector of the transistor 16 through a resistor 20. In addition, the connection point between the inductive element 14c and the capacitor 14d in the resonating circuit 14 is connected to the collector of the transistor 16 through a resistor 21.
Moreover, the oscillation frequency of the oscillator 13 is controlled by a PLL circuit 25 on the basis of channel selection data D, and, as a result of the control, a tuning voltage Vt is output from the PLL circuit 25 and is supplied to the tuning voltage terminal 4.
Next, according to the configuration described above, when a high-band television signal is received, the transistor 16 is turned on. If doing so, the anodes of the individual varactor diodes 2d, 8a, 8d, and 14a have a ground potential. In this state, the tuning voltage Vt is applied to the cathodes of the individual varactor diodes 2d, 5, 8a, 8d, 10, and 14a. In this case, when the tuning voltage falls and a low-band television signal is received, the tuning voltage is excessively lowered. For this reason, when the low-band television signal is received, the transistor 16 is turned off. If doing so, a voltage divided by the resistors 19 and 3 is applied to the anode of the varactor diode 2d, a voltage divided by the resistors 20 and 9 is applied to the anodes of the varactor diodes 8a and 8d, and a voltage divided by the resistors 21 and 15 is applied to the anode of the varactor diode 14a. Accordingly, the tuning voltage to be applied to the cathodes of the individual varactor diodes is increased, and thus a receiving frequency range can be expanded, without using an unstable low tuning voltage (for example, see Japanese Utility Model No. 3106132 (FIG. 1)).
In a related art transceiver, the same tuning voltage is applied to the varactor diodes 2d, 8a, and 8d in the tuning circuit and the varactor diode 4a in the oscillator. Accordingly, even when the resistance values of the resistors 19, 20, and 21 and the resistance values of the resistors 3, 9, and 15 are independently set, in a wide frequency range, a tuning frequency and a local oscillation frequency are rarely changed at the same frequency gap when the tuning voltage is changed. As a result, a so-called tracking error occurs.